12 And truly the spirit of expiation or reparation has always had the first and foremost place in the worship given to the Most Sacred Heart of Jesus, and nothing is more in keeping with the origin, the character, the power, and the distinctive practices of this form of devotion, as appears from the record of history and custom, as well as from the sacred liturgy and the acts of the Sovereign Pontiffs. For when Christ manifested Himself to Margaret Mary, and declared to her the infinitude of His love, at the same time, in the manner of a mourner, He complained that so many and such great injuries were done to Him by ungrateful men - and we would that these words in which He made this complaint were fixed in the minds of the faithful, and were never blotted out by oblivion: "Behold this Heart" - He said - "which has loved men so much and has loaded them with all benefits, and for this boundless love has had no return but neglect, and contumely, and this often from those who were bound by a debt and duty of a more special love." In order that these faults might be washed away, He then recommended several things to be done, and in particular the following as most pleasing to Himself, namely that men should approach the Altar with this purpose of expiating sin, making what is called a Communion of Reparation, - and that they should likewise make expiatory supplications and prayers, prolonged for a whole hour, - which is rightly called the "Holy Hour." These pious exercises have been approved by the Church and have also been enriched with copious indulgences.
Source: Miserentissimus Redemptor (Vatican.va)